1. Field of the Invention
The present invention relates to a gas sensor comprising a sensor element for detecting the concentration of a specific gas component in a measurement gas and a housing for supporting the sensor element therein, the sensor element having a rectangular solid structure of a solid electrolyte body containing a ceramic material.
2. Description of the Related Art
Among gas sensors, certain oxygen sensors have been known as an oxygen concentration detector. The oxygen sensors are used for detecting the oxygen concentration of an exhaust gas from an internal combustion engine, thereby optimally controlling the combustion state of the engine based on the detected signals, to achieve exhaust gas cleaning, fuel cost reduction, etc.
In one of such oxygen sensors, an oxygen ion-conductive solid electrolyte, such as zirconium oxide doped with calcium oxide, yttrium oxide, or the like, is used for a partition wall. Electrodes are formed on the surfaces of the partition wall to provide a sensor element. One of the electrodes is exposed to a reference atmosphere, the other is exposed to a measurement gas such as an exhaust gas, and an electromotive force generated based on the theory of oxygen concentration cell is obtained as a detection signal.
Among oxygen sensor elements, long plate elements, which have, at one end, an oxygen-detecting section to be exposed to a measurement gas such as an exhaust gas, have recently attracted much attention instead of bottomed cylinder elements. Such long plate elements can be produced and miniaturized easily. Examples of the plate elements are described in Japanese Laid-Open Patent Publication Nos. 58-153155 and 61-097562, etc.
In another oxygen sensor, an oxide such as titanium oxide is disposed on a long plate substrate to provide a sensor element, the electrical resistance of the oxide changing depending on oxygen concentration. The sensor element is exposed to a measurement gas such as an exhaust gas to detect the electrical resistance change depending on the oxygen partial pressure.
In the oxygen sensors, an oxygen-detecting section of an oxygen sensor element is exposed to a measurement gas such as an exhaust gas with high temperature, and the oxygen-detecting section is maintained in some cases at high temperature by a heater built in the oxygen sensor element to achieve sufficient oxygen-detecting function. Thus, the sensor element is subjected to thermal stress. When the sensor element is plate-shaped, the stress is concentrated at the edges, so that the element is often cracked to deteriorate its oxygen concentration-detecting function.
For example, in an oxygen sensor element disclosed in Japanese Patent No. 2786507, edge portions at the end of a plate substrate are chamfered, whereby the stress concentration is relaxed to improve the thermal shock resistance.
The conventional oxygen sensor element disclosed in Japanese Patent No. 2786507 is designed in view of relaxing the concentration of thermal stress.
However, there is a problem that, in an internal combustion engine or the like, the oxygen sensor element is brought into contact with a housing for supporting the oxygen sensor element, particularly with a supporting member for supporting a longitudinal center portion of the oxygen sensor element and a connector for supporting a rear portion of the oxygen sensor element, and the oxygen sensor element is broken by the contact. The problem has to be considered to put the oxygen sensor element into practical use in the internal combustion engine.